Heretofore, demand for a silver halide photographic light-sensitive material has been severe year by year. In addition to the demand for the enhancement of sensitivity and of high image quality (especially excellent graininess and sharpness), processing compatibility with low replenishment to reduce environmental pollution and rapid processing property for finishing in a short time. Most of these demands can be solved by attaining enhancement of sensitivity of silver halide grains. It is no exaggeration to say that the enhancement of the sensitivity of silver halide grains is the most important theme in this industry.
With regard to the enhancement of the sensitivity of silver halide grains, many studies have been conducted for a long time. More concretely, there are methods of chemical sensitization and spectral sensitization. Methods described below are known.
As a method for enhancing sensitivity by means of chemical sensitization, chemical sensitization methods such as an independent use or combination use of sulfur sensitization, noble metal sesitization (for example, gold sensitization, paradium sensitization, platinum sensitization, irridium sensitization and selenium sensitization are cited) reduction sensitization are known.
In addition, for example, methods to use as a spectral sensitization dye used in spectral sensitization, cyanine dyes such as a zeromethine dye, a monomethine dye, a dimethine dye, a trimethine dye and a merocyanine dye independently or in combination (for example, super sensitization) are known.
The above-mentioned technologies are described also in U.S. Pat. Nos. 2,688,545, 2,912,329, 3,397,060, 3,615,635 and 3,628,964, British Patent Nos. 1,195,302, 1,242,588 and 1,293,862, German Patent (OLS) Nos. 2,030,326 and 2,121,780 and Japanese Patent Publication Nos. 4936/1968 and 14030/1969.
Of the above-mentioned technologies, in a practical light-sensitive material, especially in a color light-sensitive material, a technology of spectral sensitization in which silver halides having a specific sensitivity to a blue light are sensitized by a green light or a red light is essential.
One method for obtaining a high sensitivity by means of spectral sensitization is to select an appropriate combination condition of the above-mentioned chemical sensitization method and spectral sensitization method. However, it is insufficient, by this method only, to cope with the above-mentioned requests on light-sensitive materials.
Another method is to select an appropriate sensitizing dye. Conditions to be satisfied as a sensitizing dye used for photographic materials satisfy not only that high spectrally-sensitized sensitivity can be obtained but also that there is no augmentation in fogging when the sensitizing dye is added to a silver halide emulsion, its spectral absorption properties are excellent, properties when subjected to exposure (for example, excellent latent image stability and little dependency to humidity and temperature in exposure) are excellent, the changing in terms of fogging rising, sensitivity and gradation in the course of preservation of raw samples (a light-sensitive material before being subjected to exposure and development) are small, deterioration at white background due to the remaining of the sensitizing dye in the light-sensitive material after photographic processing is not caused, and the sensitizing dye is excellent in terms of production stability and the like. However, it is extremely difficult to choose a sensitizing dye which satisfies all of the above-mentioned.
In addition, as a method for enhancing sensitivity, it is known that a super sensitization method is useful. Super sensitization is described in Photographic Science and Engineering, Volume 13, pp. 13 -17 (1969) and Volume 18, pp. 418-430 (1974) and The Theory of the Photographic Process 4th edition, page 259, published by MacMillan Inc., 1977. It is known that, by selecting suitable sensitizing dyes and super sensitizers, high sensitivity can be obtained.
Heretofore, many compounds such as stilbene, azaindene, mercaptoheterocycles, thiourea and condensed compounds between phenol and hexamethylenetetraamine have been known as a super sensitizer. For example, they are disclosed in U.S. Pat. Nos. 2,875,058, 3,340,064, 3,457,078, 3,458,318, 3,615,632, 3,695,888 and 4,011,083 and Japanese Patent Publication Open to Public Inspection (hereinafter referred to as "Japanese Patent O.P.I. Publication") No. 203447/1986.
However, from results of the experiments conducted by the present inventors, it was discovered, though the reason is not clear, that the fluctuation and deterioration of photographic sensitivity of raw samples due to natural aging is noticeable when the sensitivity is enhanced using the above-mentioned prior arts in order to satisfy the above-mentioned requests on light-sensitive materials. Storage stability of the raw sample of a photographic light-sensitive material is extremely important because uniformity of photographic performance is demanded. Accordingly, deterioration of the photographic performance after the preservation of raw samples has been a big obstacle for the practical enhancement of sensitivity. In addition, when a super sensitizer as disclosed in Japanese Patent O.P.I. Publication No. 25833/1992 is applied to photographic emulsions, development inhibition occurs in photographic processing so that request for rapid processing cannot be satisfied.
As stated above, technologies to realize enhancement of sensitivity without deteriorating various photographic properties have been demanded.